Law firms Covington & Burling and DLA Piper are also missing. Law firms Linklaters and Reed Smith LLP have signed up but fail to disclose information on their clients in violation of the registry requirements, says Alter-EU.

Over 60 percent of the companies lobbying to get the controversial Anti-Counterfeiting Trade Agreement (ACTA) ratified are also missing.

In other instances, the stated amount spent by the company on lobbying is questionable.

Alter-EU found 50 cases where companies claim they spend less than one euro per year on lobbying.

The American camera equipment producer, Panavision, is another anomaly. It spent more on lobbying than ExxonMobil, Shell and GDF Suez combined.

But Panavision has no Brussels-based office and is not affiliated with any Brussels-based lobby consultancies or think tanks, says Alter-EU.

In the US, the camera company spent $10,000 on lobbying in 2010 and nothing in 2011. In Brussels, it spends €35 million per year on lobbying.

Alter-EU believes that a mandatory register is necessary if the two EU institutions are sincere in their stated commitments to transparency.

The European Commission, for its part, said the register will come under review next year.

"The question of whether or not to make it compulsory can be looked at again then, along with any other issues arising from two years' experience with the register," institutional affairs spokesperson Antonio Gravili told this website.

In June, the commission launched a new transparency portal where the public can peruse lobbyists and identify the individuals who sit on its expert and advisory committees.

"The transparency portal brings all those tools, and lots more information besides, front and centre - where they belong," said Maros Sefcovic, the EU commissioner responsible for inter-institutional relations.